The term "water hammer" is well known to engineers, contractors, maintenance personnel and other persons engaged in the plumbing and piping industry. The destructive forces and hammer blow sounds associated with "water hammer" have caused annoyance, inconvenience and costly damage in piping systems. Water hammer is the term used to define the destructive forces, pounding noises and vibration which develop in a piping system when a column of non-compressible liquid flowing through a pipe line at a given pressure and velocity is stopped abruptly. The tremendous forces generated at the point of impact or stoppage can be compared, in effect, to that of an explosion.
When water hammer occurs, a high intensity pressure wave travels back through the piping system until it reaches a poit of some relief such as a large diameter riser or piping main. The shock wave will then surge back and forth between the point of relief and the point of impact until the destructive energy is dissipated in the piping system. This violent action accounts for the piping noise and vibration. The common cause of shock is the quick closing of electrical, pneumatic, or spring loaded valves or devices as well as the quick hand closure of valves or fixtures. The speed of the valve closure time, especially during the last fifteen percent of valve closure, is directly related to the intensity of the surge of pressure.
The water hammer shock wave travels back and forth in the piping between the point of quick closure and the point of relief at a rate of 4,000 to 4,500 feet per second. The shock wave alternately expands and contracts the piping during its travel and may cause any of the following conditions: ruptured piping, leaking connections, weakened connections, pipe vibration and noise, damaged valves, damaged water meters, loosened pipe hangers and supports, ruptured tanks and water heaters, and premature failure of other equipment and devices.
Water hammer arresters are designed to employ a cushion of gas or air to absorb and control water hammer and substantially act as a shock absorber to lessen the effect of the water hammer. Various water hammer arresters have been known in the art, including those which are chambers with fittings at the top through which a charge of compressed air can be admitted into the chamber. However, unless the charge of air or gas is kept separate from the water, the air or gas will gradually diffuse into the water and the charge will be lost. Therefore, water hammer arresters often employ pistons with various seals therearound as a separation means. Normally, brass is used as a piston material and this, and other common materials tend to corrode after long emplacement in the piping system and the seals or pistons tend to leak, enabling the gas charge to escape. Additionally, the water may contain fine amounts of sand or calcium carbonate; i.e. hard water, which may precipitate from solution and cause mechanical abrasion of the seals and eventual leakage.